beta-Defensins are small (3-5Kd in size) secreted proteins that are components of innate immunity; some are constitutively expressed, such as human beta-defensin (HBD)-1, while others, like hBD2 and -3, are inducible by cytokines or other immune response stimuli. beta-defensins are secreted preeminently by epithelial cells, and by neutrophil cells, although their secretion has been observed also in T and NK cells, and initially they were described as antimicrobial proteins; recent research has indicated that they also act as chemoattractants. Their expression is elevated in the epithelia of the mouth, tongue, digestive apparatus, and also in airways, mammary gland, liver and other organs. In particular, beta-defensins are present in saliva, and the concentration of these proteins can be very high in the oral cavity, with measured local concentration as high as 100mu g/ml, in a 100mu m-thick layer in the tongue. Therefore, it appears that beta-defensins, as important component of the innate imniunity, control the occurrence of infections in the oral cavity. The antimicrobial activity of beta-defensins is due to their ability to permeabilize bacterial membranes. Taken together, this information indicates that beta-defensins could provide a form of innate immunity against oral HIV infection that might be exploited for anti-HIV prophylaxis. In agreement, our preliminary data show that select beta-defensins, especially hBD2, inhibit R5 HIV infection in a dose dependent manner, at doses that are compatible with or below those measured in the oral cavity. In addition, our studies show that hBD2 treatment directly on the virus lowers HIV infection. This antiviral activity is reminiscent of the recently reported HIV suppressive properties of alpha defensins. However, only beta-defensins are naturally present in the oral cavity at HIV-suppressive concentrations. Therefore, our central hypothesis is that beta-defensins mediate and antiretroviral mechanism, based on inhibition of viral entry, in the oral cavity that is capable of preventing oral HIV transmission. Accordingly, we propose to a) characterize which HIV- 1 phenotypes are suppressed by hBD2. These experiments will define the broad suppressive effects of hBD2 and whether its mechanism of suppression functions in vivo b) determine the effects of hBD2 on events in the HIV-1 life cycle. In this aim we will examine and compare the effects of hBD2 on HIV entry/fusion and on intracellular steps of viral replication. The elucidation of the event(s) in HIV infection and expression that are affected by hBD2 will allow for a thorough study of its mechanism of action and for structure-function studies, thus proving basis for novel antiviral therapeutic and preventive strategies. Finally, these studies will constitute a contribution towards the understanding of the role of innate immunity in the oral cavity.